(1) Field of the Invention
The present invention relates to compositions and a method using extrusion blending which incorporates a hyperbranched polymer (HBP) and an in-situ anhydride modified hyperbranched polymer (HBP) into a biodegradable polymer in an amount, which significantly increases the impact strength and percent elongation without an unacceptable decrease in tensile strength and modulus of the biodegradable polymer. The biodegradable polymers are preferably a polyhydroxyalkanoate (PHA) polyester or a polylactic acid (PLA) polyester. The compositions can incorporate fillers.
(2) Description of the Related Art
Biodegradable polymers are moving in to the mainstream due to the nondegradable nature of conventional polymers and their exhausting petroleum sources. The polyhydroxyalkanoates (PHAs) polymers are biodegradable polyester polymer of a 3-hydroxyalkanoic acid containing 3 to 14 carbon monomers. Typically the commercial PHA's are polyhydroxybutyrate (PHB) and/or polyhydroxybutyrate-valerate (PHBV), which are derived from bacterial fermentations. The monomers and polymers can also be produced chemically. Polylactic acid (PLA) is polyester based upon lactic acid, which is a three-carbon monomer. Both PHA and PLA are linear polymers, which are chemically related to each other but the polymers have different physical properties.
Polyhydroxyalkanoates (PHAs) are the biodegradable polyesters commercially produced by several bacteria as intercellular carbon and energy storage materials in their cell (Hocking, P. J., et al., “Chemistry and Technology of Biodegradable Polymers”, 1st edition, Ed. Griffin, G. J. L., Chapman and Hall, Glasgow, p. 48.2. (1994)). PHAs have attracted lot of attention due to their environmentally friendly nature and biodegradability. Polyhydroxybutyrate (PHB) and poly (3-hydroxybutyrate-co-3-hydroxyvalerate), PHBV (FIG. 1) are the two most common types of PHA. PHB is a homopolymer having steroregular structure with high crystallinity. Its inherent brittleness and thermal instability impedes its commercial application (deKoning G. J. M., and Lemstra, P. J., Polymer, 34, 4089 (1993)). PHBV is a copolymer in which 3-hydroxyvalerate (HV) units are incorporated in the PHB backbone during the fermentation process. PHBV has improved flexibility and toughness over PHB. Presently, PHBV having a HV content below 15 mol % are commercially available, while large-scale production of PHBV having higher HV content is presently not commercially viable due to the surprisingly high production cost (Fei, B., et al., Polymer, 45, 6275-6284 (2004)). The available PHBV (having a HV content of less than 15 mol %) have a low toughness and elongation at break.
In general, these natural PHA polymers have similar properties to that of polypropylene (PP). They have potential to replace polypropylene and other conventional petroleum based polymers if the PHB and PHBV based materials can be developed with a balance of properties such as stiffness and toughness. Poly(lactic acid) (PLA) is a linear aliphatic polyester (FIG. 2). PLA is gaining a lot of interest due to its biodegradability, biocompatibility and renewable resource based origin. Polylactide can be prepared through the ring-opening polymerization of lactide, of which there are two commonly used isomers (L-lactide and D-lactide)(Anderson, K. S., Lim, S. H., Hillmyer, M. A., “Toughening of Polylactide by Melt Blending with Linear Low-Density Polyethylene”, Journal of Applied Polymer Science, 89, 3757-3768 (2003). The major drawbacks of PLA are its low elongation at break, impact strength, heat deflection temperature and low melt strength. One way to overcome the brittleness of PLA is through plasticization. The numbers of plasticizers have been used with appreciable success. The plasticizers reported in literature (Labrecque et. al “Citrate Esters as Plasticizers for Poly (lactic acid).”, Journal of Applied Polymer Science, 66(8),1507-1513, (1997),Martin et. al “Poly (lactic acid): Plasticization and properties of biodegradable multiphase systems.”, Polymer, 42, 6209-6219, (2001), Jacobsen et. al, “Filling of poly (lactic acid) with native starch.”, 36 (22), Polymer Engineering and Science. 2799-2804, (1996), Jacobsen et. al, “Plasticizing Polylactides—the effect of different plasticizers on the mechanical properties.”, Polymer Engineering and Science, 39(7), 1303-1310 (1999)) are citrate esters, 1,2-propylene glycol, glycerol, poly-(ethylene glycol), glucose monoesters, and fatty acids. But the major drawback of these plasticizers is their low thermal stability. In long-term use, the plasticizers have problems of leaching out, which result in embrittlement of PLA. In order to get film grade PLA sometimes 20 to 30 wt. % of plasticizers may be needed to achieve the desired flexibility. The leaching of plasticizers also leads to problems of migration. The migration of plasticizers is a critical problem when considering the application of plasticized PLA in food packaging.
Plasticization of polyhydroxyalkanoates especially PHB is done to overcome its brittleness. The common plasticizers reported in literature (Baltieri et. al. “Study of the influence of plasticizers on the thermal and mechanical properties of poly(3-hydroxybutyrate) compounds” Macromol. Symp.,197, 33-44, 2003) are dioctyl phthalate(DOP), dioctyl adipate (DOA),triacytyl glycerol (TAG),and poly adipate(PA). These plasticizers did not improve the flexibility of PHB remarkably. They also lower the properties of polymers such as modulus, strength and gas barrier.
PHB is blended with rubber to improve its toughness( Parulekar et. al “Biodegradable Toughened Polymers from Renewable Resources: Blends of Polyhydroxybutyrate with Epoxidized Natural Rubber and Maleated Polybutadiene” , Green Chemistry,8(2), 206-213, (2006). But these blends have drastically low modulus and strength. To improve compatibility between rubber and PHB additional compatibilizers are required.
PHB is also blended with other high impact biodegradable polymers such as polycaprolactone (PCL) (Shuai et. al. “Miscibility of block copolymers of poly(ε-caprolactone) and poly(ethylene glycol) with poly(3-hydroxybutyrate) as well as the compatibilizing effect of these copolymers in blends of poly(ε-caprolactone) and poly(3-hydroxybutyrate)”, Journal of Applied Polymer Science,80 (13), 2600-2608, 2001), and polybutylenesuccinate (PBS) (Qiu et. al. “Poly(hydroxybutyrate)/poly(butylene succinate) blends: miscibility and nonisothermal crystallization”, Polymer, 44(8), 2503-2508, 2003). But incompatibility between two different polymers is a big problem. We also have to add such petroleum based biodegradable polymers in large quantity, which reduced the wt. fraction of renewable/biobased PHB in the overall blend composition.
PLA is blended with rubber to improve its toughness (Jin et. al. “Blending of poly(L-lactic acid) with poly(cis-1,4-isoprene)”, European Polymer Journal, 36(1), 165-169, 2000).But the incompatibility between two polymer phases is a major drawback.
PLA is also blended with other high impact biodegradable polymers such as polycaprolactone (PCL) (Broz et. al., “Structure and mechanical properties of poly(d,l-lactic acid)/poly(e-caprolactone) blends”, Biomaterials, 24, 4181-4190, (2003). But incompatibility between two polymer phase is again a major problem.
Hyperbranched polymers are relatively new materials in the field of polymers. Their uniqueness lies in their cavernous interior and nano-scale dimensions (M. Seiler, “Dendritic Polymers-Interdisciplinary Research and Emerging Applications from Unique Structural Properties”, Chem. Eng. Technol., 25,3, (2002)). Researchers in Australia are able to improve the fracture toughness of natural fiber reinforced PLA composites modified with hyperbranched polymers (Wong, S., et al., Macromolecular Material and Engineering, 289, 447-456 (2004)). They have used solvent casting methods to treat PLA with hyperbranched polymers. Hyperbranched polymer is hydroxyl functional aliphatic polyester, having a tree like macromolecule structure in which a polyalcohol is a core from which other multifunctional compounds as repeating units extends. This forms a core-shell structure having large number of hydroxyl group at the periphery (Hyperbranched polymers-Unique design tools for multi property control in resins and coatings, Bo Pettersson, Perstorp Polyols, http://perstorp.com/upload/hyperbranched_polymers.pdf).
U.S. Pat. Nos. 5,418,301 and 5,663,247, and published Application U.S. 2005/0240000 A1 describe hyperbranched polyester polymers. These patents and application are incorporated herein in their entireties. They are not disclosed to be useful with the PHA and PLA polymers. FIGS. 1 and 2 show the PHA and PLA polymers.
The following references are illustrative of the prior art in polylactic acid.                1. Anderson, K. S., Lim, S. H., Hillmyer, M. A., “Toughening of Polylactide by Melt Blending with Linear Low-Density Polyethylene”, Journal of Applied Polymer Science, 89, 3757-3768 (2003).        2. Labrecque, L. V., Kumar, R. A., Dave, V., Gross, R. A., McCarthy, S. P.” Citrate Esters as Plasticizers for Poly (lactic acid).”, Journal of Applied Polymer Science,66(8), 1507-1513 (1997).        3. Martin, O., Averous, L., “Poly (lactic acid): Plasticization and properties of biodegradable multiphase systems.”, Polymer, 42, 6209-6219 (2001).        4. Jacobsen, S., Fritz, H. G., “Filling of poly (lactic acid) with native starch.”, Polymer Engineering and Science, 36 (22), 2799-2804 (1996).        5. Jacobsen, S., Fritz, H. G., “Plasticizing polylactides—the effect of different plasticizers on the mechanical properties.”, Polymer Engineering and Science, 39(7), 1303-1310 (1999).        